1. Field of the Invention
This invention generally relates to wireless communications. More particularly, the present invention relates to a novel and improved wireless communication system and method capable of supporting communications in any of two or three distinct frequency bands.
2. Description of Related Art
Wireless communication devices sometimes include dual-band transmitters/receivers so that they can be used with various communication services operating on two distinct frequency bands. For example, older analog cellular systems, conforming to the Advanced Mobile Phone Service (AMPS) standard, utilize frequency-modulation and operate in a frequency range of approximately 869.04-893.97 MHz. In contrast, more advanced digital systems, such as Personal Communication Systems (PCS), employ phase-modulation schemes operating in a frequency bandwidth of 1930-1990 MHz frequency bandwidth. As shown in FIG. 1, these services, and their associated frequency bands, can be accommodated by a single communication receiver infrastructure.
FIG. 1 depicts the architecture of the receiver subsystem 100 operating in a dual-band wireless communication system. Suppose, for example, that the two bands comprise Cellular and PCS services. The initial RF stage for receiver subsystem 100 includes an antenna for each of the two bands. The Cellular services antenna 110 is properly matched to operate in the 869.04-893.97 MHz frequency band, while the CS antenna 130 operates in the 1930-1990 MHz band. Antennas 110, 130 are coupled to RF front-end bandpass filters 112, 132, respectively, in order to minimize out-of-band energy contributions on the received Cellular and PCS signals. The respective outputs of front-end bandpass filters 112, 132 are coupled to Low-Noise Amplifiers (LNAs) 114, 134, so as to maintain the signal-to-noise ratio of the received Cellular and PCS signals. LNAs 114, 134 are coupled to bandpass filters 116, 136, respectively, to further remove unwanted energy on the received Cellular and PCS signals. The bandpass filters 114, 134 then attach to frequency mixers 118, 138, respectively, in order to down-convert the received Cellular and PCS signals from RF signals to IF signals.
The down-conversion process translates the frequencies of the received signals to lower IF frequencies so as to reduce the operating frequency of the receiver subsystem 100 and improve its performance. Generally, the down-conversion process mixes (i.e. multiplies) the received signal with a local reference signal having a frequency supplied by a phase-locked loop (PLL) having a local voltage-controlled oscillator (VCO). The PLL ensures a precise mixer frequency. The VCO frequency is chosen such that the mixing of the received and reference signals yields a desired reduced frequency component within the IF range.
In this case, the down-conversion employs VCOs 154, 158 to supply reference frequency signals with bandwidths corresponding to the received Cellular and PCS signal bandwidths. For example, VCO 154, generates a frequency signal at a bandwidth of 954.42-979.35 MHz and is mixed with the received Cellular signal which, after passing through a low-pass filter 120, renders a Cellular common IF signal operating at a resultant frequency of 85.38 MHz. Similarly, VCO 158, generates a frequency signal at a bandwidth of 1719.62-1779.62 MHz which mixes with the received PCS signal and, after low-pass filtering 140, produces a PCS common IF signal at 210.38 MHz.
It is important to note that, in order to operate the wireless communication system in dual-band, the conventional receiver subsystem 100 must include two VCOs (e.g., 154, 158), where each VCO is tuned to a band of frequencies corresponding to each band. In addition, two distinct common IF frequencies are generated, requiring different post receiver subsystem processing for each common IF.
With the advent of the Global Positioning System (GPS) it has become desirable to integrate a GPS receiver into a hand-held wireless communication device so that it can automatically send to a base station data indicating the location of the hand held device. To support GPS services, however, conventional wireless communication systems, operating in dual-band, have to convert to a tri-band operation in order accommodate the additional frequency band germane to GPS applications.
For example, the front-end of the conventional receiver subsystem 200 employs antenna 210 capable of receiving Cellular frequencies at 869.04-893.97 MHz, antenna 230 capable of receiving PCS frequencies at 1930-1990 MHz, and antenna 250 capable of receiving GPS frequencies at PCS frequencies at 1575.42 MHz. The received signals are front-stage filtered (i.e., 212, 232, 252), low noise amplified (i.e., 214, 234, 254), band-pass filtered (i.e., 216, 236, 256), and mixed (i.e., 218, 238, 258) in a manner similar to the conventional systems of FIG. 1.
With respect to the Cellular received signals, VCO 274 is configured to supply a reference signal with a frequency sub-band of 954.42-979.35 MHz to mixer 218 in order to down-convert the received Cellular signal to the common IF frequency of 85.38 MHz.
With respect to the PCS received signals, VCO 278 is configured to supply a reference signal with a frequency sub-band of 1719.62-1779.62 MHz to mixer 238 in order to down-convert the received PCS signal to the common IF frequency of 210.38 MHz.
With respect to the received GPS signal, VCO 282 is configured to generate a reference signal with a frequency of 1530.42 MHz which is supplied to mixer 258 to down-convert the received GPS signal to the common IF frequency of 45 MHz.
Such systems would have to, therefore, include three oscillators, each operating at a specific band that corresponds to a particular service. That is, a tri-band communication system must incorporate a first VCO (218), for accommodating the bandwidth for Cellular services and the generation of a Cellular common IF of 85.38 MHz; a second VCO (238), for accommodating the bandwidth for PCS services and the generation of a PCS common IF of 210.38 MHz; and a third VCO for accommodating the bandwidth for GPS services and the generation of a GPS common IF of 45.0 MHz.
As shown in the conventional dual-band and tri-band systems of FIGS. 1 and 2, the VCO frequency bands correlate to the operating frequency band of each service. With this said, we note that VCOs are neither one-size-fits-all nor interchangeable. Thus, wireless communication devices designed to operate with these services, such as telephonic equipment, require a different VCO in their receiver subsystem for each band of service subscribed to. In particular, wireless phones having tri-band capabilities, such as Cellular, PCS, and GPS would require three VCOs, one for each service. Clearly, it is both, inefficient and cumbersome to incorporate a separate VCO and associated PLL circuit for each desired wireless communication service. Moreover, it is equally inefficient and awkward to process a distinct common IF frequency for each service. This is especially crucial given the design limitations on wireless phones (i.e., size, weight, etc.), where such inefficiencies can directly translate into increased circuit complexity.
For the foregoing reasons, a need exists for a system and method that effectively supports wireless communications operating on two or three distinct frequency bands while only utilizing one or two oscillating devices, respectively.